The detail we followed had both sides of the ditch and top. So you'd have someone hold the sides upright as you backfill the trench. Then cover the top of the trench with foam then cover the foam. The foam ends up breaking up and it's not very good. The heat trace covers that weakness.FWIW, the fact that it only comes up in UPC Chapter 13 on Healthcare Facilities means it's not generally required. Although it still may be a good idea.
Shallow frost protected foundations use a wing of insulation outside the building footprint to raise the frost line under the foundation. I wonder if a strip of buried insulation board on top of the sewer lateral would work similarly. It might need to be impractically wide to be effective, though.
This was an engineered design.